Field of the Invention
The invention is directed to multicore optical waveguides or fibers. More specifically, the invention is directed to methods of inscribing gratings in multicore optical waveguides or fibers in a manner that reduces variations of the strength of gratings such as Bragg gratings within a core, between different cores, or the like.
Description of Related Art
Exposure of a multicore fiber for purposes of, e.g., inscribing gratings such as Bragg gratings, is conventionally done from one azimuthal direction. The fiber/longitudinal axis is perpendicular to the drawing plane, which is the transverse plane (see FIG. 8). Note that the rays incident on the fiber are usually a superposition of at least two beams with different wave vector components in the direction of the fiber axis, e.g., generated by the shown phase mask that is parallel to the fiber.
Especially in fibers that are not azimuthally invariant (e.g., multicore or microstructured fibers), the transverse distribution of the refractive index change that forms a Bragg grating depends on the azimuthal direction of the inscribing actinic (UV) radiation, see FIG. 1: The lensing effect caused by the curved interface between the fiber and the surrounding lower-index material (usually air) makes the grating in a certain core stronger if this core is on the remote side of the fiber (with respect to the incoming beam). This effect is also evident in the slow sinusoidal grating strength variation in FIG. 2, which results from twisting of the offset core into different positions into and out of the remote side of the fiber. Similarly important is the shadowing effect due to other cores or transverse inhomogeneities that can shadow or change the path of the incident actinic radiation, giving rise to the sharp spikes in FIG. 2. These huge variations (both maxima and minima) of the grating strength can substantially reduce the yield in the grating fabrication process.
For some types of fibers, for instance, multicore fibers with the cores arranged in a hexagonal array, it is possible to find at least one optimal azimuthal angle such that the variance of the grating strengths due to shadowing and lensing is minimized in the different cores of a multicore fiber. For twisted multicore fibers, this optimum angle depends on the location z along the fiber, making it impossible to globally minimize the grating strength variations if the actinic radiation source comes from only one azimuthal angle.